1. Field of the Invention
The present invention relates generally to an electronic ballast suitable for use with gaseous discharge lamps and more particularly to a circuit for correcting the power factor within a fluorescent lamp system.
2. Background Art
Fluorescent lamps usually require a ballast circuit for producing a direct current signal with a high voltage amplitude for effective operation of the fluorescent lamp. Ballasts of this type are typically separated into two broad categories, the first being of the electromagnetic type while the second is of a true electronic form. In this last particular type, it has been found that electronic ballasts inherently had power factors of 50% or less. Especially those associated with so-called "task" lighting which draw 30 watts or less power. In such small lamps the power factor is a numerical value that expresses the angular relationship found between load current and line voltage. If load current and the line voltage are in phase timewise, the power factor is 100%. When the percentage is substantially lower, a greater angle exists in between the two. Where a low power factor is present, this arrangement typically results in a substantially higher current draw from the power line for a given output lamp load. More importantly, power factors are frequently the cause of an unbalanced current loading. If such a situation develops, the neutral line could then become overloaded and burn out.
A number of schemes have been used to correct for the out-of-phase relationship between load current and line voltage in electronic ballasts. Most schemes employ some kind of an expensive saturable inductor in a "buck" or "boast" mode. Such an arrangement is taught in U.S. Pat. No. 4,109,307. Other approaches to the particular problem are diclosed in U.S. Pat. Nos. 4,415,839, 5,008,599, and 4,647,817. None of these provide the satisfactory and low cost approach to the solution of the above noted problem found in the present invention.